This invention relates to modulation techniques in digital communications and, in particular, to an apparatus for generating minimum-shift-keying (MSK) digital modulation signals.
MSK is a well established form of digital modulation in digital communication systems. With MSK, different data types as, for example, data type "0" (commonly referred to as a space) and data type "1" (commonly referred to as a mark) are associated with different frequencies, e.g., f.sub.1 and f.sub.2, which are transmitted as indicative of the data. It is known that to obtain the desirable spectral efficiency offered by MSK, the generating apparatus must generate the frequencies f.sub.1 and f.sub.2 with a specific relationship. In particular, the two frequencies must be generated so that they differ by one-half the modulation or data rate f.sub.d.
In the prior art MSK apparatuses, a variety of serial or parallel modulation schemes have been used. These schemes require relatively complex oscillator arrangements and quadrature networks, as illustrated by the schemes disclosed by Peebles, P.Z., Digital Communication Systems, Prentice Hall, ISBN 0-13-211970-6, pp 274-291. Conceptually more simplified arrangements are disclosed by Amoroso, F., and Kivett, J.A., in an article entitled Simplified MSK Signaling Technique, IEEE Trans. Comm., Vol. COM-25, No. 4, Apr. 1977, pp. 433-441. These arrangements, however, require a special matched filter, typically realized by surface acoustic wave technology and, thus, not readily available to everyone. Even more simplified arrangements employ a voltage controlled oscillator modulated by the data and having a frequency deviation of plus or minus one-quarter of the data rate. Such arrangements, however, have the usual problems of deviation linearity and stability.
It is, therefore, a primary object of the present invention to provide a simple but reliable apparatus for generating MSK signals.
It is a further object of the present invention to provide an apparatus for generating MSK signals using digital techniques, allowing adjustment-free operation and flexibility with respect to data rate.